A cellulose acylate film is formed from cellulose acylate. In the production of the cellulose acylate film, cellulose triacetate (hereinafter TAC) is used as cellulose acylate for forming a TAC film, while the averaged acetylation degree of TAC is in the range of 57.5% to 62.5%. The TAC film is used as a film base of a film material, such as a photosensitive material, since having strength and inflammability. Further, the TAC film is excellent in optical isotropy, and therefore used as an a protective film in a liquid crystal display whose market becomes larger in recent years, and the like.
The TAC film is usually produced by a solution casting method, in which the produced film is more excellent in physical properties such as optical properties and the like than other film production method such as a melt extrusion method and the like. When it is designated to perform the solution casting method, polymer is dissolved in a mixture solvent in which dichloromethane or methyl acylate is the main solvent component, so as to prepare a polymer solution (hereinafter, dope). The dope is cast from a casting die onto a support while a bead of the cast is formed between the casting die and the support. Thus a casting film is formed from the dope on the support. When the casting film has a self-supporting property, the casting film is peeled as a wet film from the support and dried to be a film. Thereafter the film is wound up. (cf: Japan Institute of Invention and Innovation (JIII) Journal of Technical Disclosure No. 2001-1745)
Further, in the melt extrusion method, in order to obtain the produced film having an adequate optical isotropy, the stretch of the film in a widthwise direction and the relaxation thereof in a longitudinal direction are made (see, Japanese Patent Laid Open Publication No. 2000-309051). By performing both of the relaxation and the stretch, the birefringency of the film is controlled. The film having the birefringency is preferably used for an optical film to be provided in a liquid crystal display. The stretch and relaxation of the film is made by a tenter device (also called tenter dryer). Further, in order to obtain the adequate birefrigency of the film, it is effective to adjust the increase and decrease of the stretch ratio and control the film temperature. And further, in the solution casting method, it is especially effective to adjust the content of the remaining solvent in the film.
By the way, it is necessary for the optical functional film, especially the protective film of the polarizing filter to provide with not only high transparency and high strength but also high heat resistance. If the heat resistance of the protective film is not enough, the shrinkage and the deterioration of the protective film easily occurs under the high temperature and high humidity, and otherwise the deterioration and the like of the adhesive agent layer between the protective film and a glass plate of a liquid crystal cell. The shrinkage, the deterioration and the like causes the peeling of the protective film from the glass plate.
Therefore, there is a method in which other polymer is used than TAC in order to increase the moisture resistance and the heat resistance of the film to be produced by the solution casting method. For example, for the acylation of the cellulose, as described in Japanese Patent Laid Open Publication No. 2001-188128, the acylations of acetyl group (—CO—CH3) and propionyl group (CO—C2H5) are made such that cellulose acylate propionate (hereinafter CAP) may be produced, and then the CAP is used as a raw material of the film. CAP has a longer side chain acyl group than TAC, and thus has a larger lipophilic property (nonhydrophilic property). Therefore the dissolubility to the organic solvent becomes larger and the production of the dope becomes easily.
The liquid crystal display of VA (vertically aligned) made with the optical compensation property is used in recent years. In order to realize a high speed responsiveness of the liquid crystal display, a technique for narrowing a gap (cell gap) between glass plates for sandwiching liquid crystal molecules is used. This technique is effective in view of making optical compensation since the in-plane retardation (Re) of the film becomes larger. However, the thickness retardation (Rth) also increases at the almost same ratio as Re, and therefore the thickness retardation (Rth) becomes too large in comparison to the cell gap. Thus the optical property of the liquid crystal display becomes worse.
In the Publication No. 2000-309051, the stretch in the widthwise direction (TD direction) and the relaxation in the longitudinal direction (MD direction) are made at the same time, and thus the optical property of the film, especially the control of the refractive index is described. However, the object of performing the above method is to reduce the occurrence of the bowing phenomena of the film, and the adjustment of the birefringency is not described.
In the Publication No. 2001-188128, the protective film can be produced so as to have high heat resistance. In this case, however, the film is produced from CAP under the almost same conditions as from TAC, and therefore the in-plane retardation Re becomes larger. Thus the thickness retardation Rth becomes too large. Further, since if the stretch ratio becomes too large, the film rupture and the film bleaching causing the bad optical property occurs. Therefore, there is an upper limit of the stretch ratio. In order to resolve these problems, the addition of the refractive index controller is supposed. However, the amount of adding the refractive index controller is limited such that the optical property of the film may not be bad.
An object of the present invention is to provide a solution casting method for producing a film in which both of the in-plane retardation is increased and the thickness retardation is decreased.